Titanium oxide is used in large quantity in various fields as a component of coating compositions, a delustering agent for chemical fibers, printing inks, cosmetics, etc. Processes for producing titanium oxide on an industrial scale are generally classified into two processes, namely, the sulfate process and the chloride process, and the former process has been mainly employed to date.
The sulfate process generally comprises the steps of (1) dissolving a titanium slag or raw ilmenite ore into sulfuric acid to obtain a titanium sulfate solution, (2) adding waste iron or waste aluminum to the titanium sulfate solution to chemically reduce ferric ion contained as an impurity in the solution to the divalent (ferrous) state in order to prevent precipitation of iron and to increase the degree of whiteness of the titanium oxide product, followed by cooling the solution to precipitate and remove ferrous sulfate, (3) heat-hydrolyzing the titanium sulfate solution from which ferrous sulfate had been removed, followed by precipitating hydrous titanium oxide, which is then filtered and washed, and (4) then calcining the washed hydrous titanium oxide at 800 to 1,100.degree. C. to obtain anhydrous titanium oxide.
In the sulfate process described above, sulfuric acid solution is discharged in large quantity mainly in step (3) above. Treatment of this waste sulfuric acid has become a serious problem with respect to efficient use of resources, protection of the environment, etc. In the sulfate process, sulfuric acid is used in a unit amount (an amount to produce 1t of TiO.sub.2) of about 3.5-5.0 t, and 1.0-1.5 t of the sulfuric acid is fixed as ferrous sulfate (step (2) above) in the case of production from ilmenite ore, with the remaining sulfuric acid being discharged as waste sulfuric acid.
The waste sulfuric acid contains a large amount of iron in addition to titanium, and further contains ions of titanium, manganese, aluminum, magnesium and other elements. Although part of the waste sulfuric acid is reused as ammonium sulfate, most of the waste sulfuric acid is presently disposed of in a landfill as gypsum, or discharged into the sea after first being neutralized. Thus, the waste sulfuric acid is treated at enormous cost.
On the other hand, investigations are being conducted relating to the reuse of recovered waste sulfuric acid for dissolving ores. However, the removal of divalent (ferrous) iron contained in the waste sulfuric acid is required, and the recovered waste sulfuric acid must be further concentrated. Since the solubility of iron in sulfuric acid is high, it is considered that a cost-saving and efficient method for removing ferrous ion contained in the waste sulfuric acid comprises oxidizing the ferrous (divalent iron) ion to ferric (trivalent iron) ion, and then removing the ferric ion by solvent extraction.
Conventional techniques for removing ferrous ion, however, are disadvantageous from a practical standpoint. For example, although ferrous ion is oxidized to ferric ion by exposure to air, the reaction proceeds extremely slowly and is inefficient. Ferrous ion is also oxidized by nitrogen oxides to obtain ferric ion, but the oxidation reaction yields nitric acid in the solution. The nitric acid corrodes the apparatus and disadvantageously oxidizes solvents and extractants used in the subsequent solvent extraction.
In the case where an aqueous hydrogen peroxide solution is used as an oxidizing agent, the divalent iron ion is quickly oxidized to the trivalent state, but is disadvantageous in that the reaction takes place vigorously and is dangerous because a large quantity of aqueous hydrogen peroxide solution is required. Furthermore, aqueous hydrogen peroxide solution is expensive, and the remaining hydrogen peroxide decomposes during solvent extraction, to thereby interfere with the solvent extraction.